Polybutene-1 is widely used for production of flexible pipe, pipe fittings and various injection molded articles. Flexible pipe is generally manufactured by extrusion. However, difficulties arise in joining polybutene-1. Joining polybutylene articles has been accomplished by conventional mechanical means such as threaded joints and flange connections, by thermal means such as flame or electrical resistance welding, or fusing with a welding sleeve, where external pressure is applied to articles being joined at predetermined times during the welding process so that the articles in the welding sleeve are held in contact throughout the operation. See U.S. Pat. No. 3,506,519.
Fusion welding is a commonly employed method of joining thermoplastic materials. Heat is applied or generated by any of a wide variety of methods at the two surfaces to be joined until they become molten. The surfaces are then brought together while they are still in the molten state, whereafter polymer chain segment interdiffusion proceeds and the interface between the two original surfaces becomes diffuse or altogether disappears. Subsequent cooling then restores the material strength and the bonded region may have strength equal to that of the surrounding material. The strength of amorphous polymers develops upon vitrification or passing below the glass transition temperature of entangled chains whereas that of semicrystalline polymers develops by the crosslink-like network of crystalline domains. It is common practice to join polybutylene by fusion welding.
Non-crystalline, amorphous polymers such as polystyrene or polycarbonate can be readily solvent welded to form strong joints simply by the application of an appropriate solvent to the surface, followed by the bringing together of two such surfaces for a certain length of time. This process generally occurs without the application of heat, by slow solvation of the amorphous polymer at the two surfaces which increases the chain segment mobility, effectively lowering the glass transition temperature locally to ambient temperature. Subsequently, chain entanglement across the interface can follow and diffusion of solvent into the surrounding material then gradually restores the polymer to its original properties. This process leaves the two surfaces effectively welded together with very high strength, often approaching the full strength of the material. This process is analogous to fusion welding but has the advantage that it can be accomplished more leisurely and at ordinary temperatures.
In contrast to non-crystalline, amorphous polymers, however, semicrystalline polyolefins such as polypropylene, polyethylene and polybutylene, generally cannot be successfully joined by solvent welding. In these materials the amorphous polymer segments are constrained by participation in nearby crystallites. The response of this type of crystalline-amorphous structure to solvents is analogous to that of a crosslinked network. Swelling of the amorphous phase by good solvents can occur, but unless the crystallites dissolve or are melted, the chain segments are not free to diffuse across the interface and to entangle into the adjacent surface. In the process of fusion welding, on the other hand, the crystallites are not longer present and intersegment diffusion is facile. There has been a long felt need in the thermoplastic industry for a method of solvent welding semicrystalline polymers.
U.S. Pat. No. 4,113,804 claims various polybutene-1 compositions that can be solvent welded and that can be used with solvent to form adhesives for those compositions. However, all of these compositions must contain EPDM interpolymer elastomer of ethylene, polyene and monoolefin, an amorphous, non-crystalline ingredient. Column 3, line 50 of U.S. Pat. No. 4,113,804 reveals that the amorphous elastomer is present in an amount which enhances the adhesive qualities of the polybutene-1, indicating clearly its role in affecting weldability. Column 3, line 47 reveals that the amorphous elastomer may be present in amounts greater than the polybutene-1. Further, U.S. Pat. No. 4,113,804 in column 12, line 10 discloses more rapid crystallization of the aforementioned blends than occurs with pure polybutene and claims that as an advantage. Still further, U.S. Pat. No. 4,113,804 reveals in column 9, line 24 that, prior to welding, the plastic article may be treated with an oxidizing material but nowhere mentions the time window immediately after solidification from the melt as a necessary parameter of the invention. No examples are given showing the effectiveness of solvent welding of these amorphous EPDM compounds.